Gated imaging systems are known in the art and have found use in many areas of technology such as in law enforcement, the maritime and fishing industries, medical imaging, transportation and the military. Such systems usually employ a light source, such as a laser, which produces laser pulses aimed at an object or scene of observation. A sensor, such as a camera, is timed in coordination with the emitting of the laser pulses to open and close its shutter to receive reflections of the laser pulses from the object or scene. Received reflections can then be used to produce an image of the object or scene as well as three-dimensional information of the object or scene.
Laser pulses to be used in a gated imaging system can be produced in many different ways. For example, a laser may be operated in a continuous wave (herein abbreviated CW) mode with a shutter placed at its output. By opening and closing the shutter at various speeds or frequencies, the CW laser can be made to produce laser pulses. Techniques such as mode-locking, Q-switching and gain-switching can also be used to produce laser pulses, as is known in the art. Light pulses in general, and laser pulses in particular, are substantially bursts of electromagnetic energy having a finite duration with a substantially clear beginning and end in time. In order to establish a common terminology for referring to laser pulses and light pulses in this application, reference is now made to FIG. 1, which is a graph illustrating various sections of a light pulse, generally referenced 10, as is known in the prior art. Graph 10 includes an X-axis 12 and a Y-axis 14. Graph 10 is merely schematic and therefore does not show any numerical values. X-axis 12 represents time and Y-axis 14 represents current, where current is a measure of flow and is used in FIG. 1 to represent the energy of a laser pulse 16. It is noted that voltage is not shown in FIG. 1 but is implied. Laser pulse 16 is an idealized laser pulse and includes a number of different sections such as a rise time 18, a pulse width 20, a fall time 22 and a delay time 24. Rise time 18 represents an increase in intensity of the current from about 10% to 90% of the intensity of the current at pulse width 20. Fall time 22 represents a decrease in intensity of the current from about 90% to 10% of the intensity of the current at pulse width 20. A plurality of dotted lines 30 demarcates the beginning and end of each section of laser pulse 16. The combined time duration of rise time 18 (in this case, starting from 0% of the intensity of the current at the pulse width), pulse width 20, fall time 22 and delay time 24 forms a period 28 of laser pulse 16. After period 28, a second laser pulse 26 begins to form. Period 28 substantially represents the time duration of the formation of a pulse until the formation of a subsequent pulse, as shown in FIG. 1.
Laser pulse 16 begins at an arbitrary current A0, which may be zero or some other number representing a current at which no laser pulse is generated. During rise time 18, the current of laser pulse 16 begins to climb from A0 to A1, with A1 representing the desired energy and intensity at which laser pulse 16 is supposed to be at. Rise time 18 therefore represents the time duration required to generate the requisite energy for laser pulse 16. Once 90% of A1 has been reached, rise time 18 ends and pulse width 20 begins. Pulse width 20 is substantially the laser pulse referred to when referring to laser pulses and is the full powered laser pulse which is reflected off of an object or scene of observation and is received by a sensor or camera for gaining information about the object or scene. It is noted that the sections of the laser pulse in rise time 18 and fall time 22 may also be received by a sensor or camera for gaining information. Fall time 22 is the time duration during which the current of laser pulse 16 descends from 90% of A1 to about 10% of A1 (i.e., in the direction of A0), thus representing the termination of laser pulse 16. Delay time 24 represents the time duration between the end of laser pulse 16 and the next laser pulse, such as second laser pulse 26. Rise time 18 can also be referred to as the time duration during which a laser pulse is generated, fall time 22 can also be referred to as the time duration during which a laser pulse is terminated and pulse width 20 can also be referred to as the time duration during which a laser pulse is maintained.
As can be seen in FIG. 1, various characteristics of laser pulses can be described based on the ratio of the time duration of the various sections of laser pulse 16. For example, the duty cycle of a pulse, such as a laser pulse represents the ratio of substantially how much time in a period is a laser pulse generated and maintained as compared to the delay time when no laser pulse is produced. Duty cycle can in general be used to describe the ON/OFF ratio of any device having an ON state and an OFF state. A high duty cycle laser pulse is one in which there is more pulse width than delay time whereas a low duty cycle laser pulse is one in which there is more delay time than pulse width. This can include a scenario where there is no pulse width but substantially only a rise time and a fall time. Both the rise time and the fall time of laser pulse 16 can be characterized in terms of how quickly or slowly a laser pulse is generated and terminated.
As mentioned above, gated laser systems have been in use in many industries for a variety of purposes. Such systems usually include at least a laser and an electronic circuit or processor. The laser is designed to substantially produce a laser pulse having a specific pulse width. The laser pulse can either be generated by using a known laser technique for generating laser pulses, such as by mode-locking or Q-switching, or via a CW laser outfitted with a shutter which is opened and closed, thereby only allowing pulses of the generated laser light to exit. The electronic circuit or processor is used to time and synchronize the generation of laser pulses, for example by deciding when the shutter on the laser is opened or closed in a CW laser. It is in this respect that such laser systems are termed “gated” laser systems, the “gating” referring to the timing and synchronization of when laser pulses are produced. Examples of known uses of gated laser pulses are etching and marking products, performing surgery, estimating distances and determining the speed of moving objects.
Gated imaging systems are similar to gated laser systems, however such systems also include at least one sensor for sensing reflections of the generated laser pulses off of objects in a scene. In such systems, the sensor also includes a shutter which can be opened or closed for only receiving reflections of generated laser pulses at specific times. The electronic circuit or processor in such systems controls and synchronizes the generation of laser pulses as well as the opening and closing of the shutter on the sensor. In general, gated imaging systems are designed to image specific distances in front of such systems (i.e., a specific depth of field). In such systems, a laser pulse is generated. During the generation of the laser pulse, the shutter of the sensor is closed and does not accumulate any reflected photons of the laser pulse. As the speed of light is known, the opening of the shutter is timed to remain closed until photons from the emitted laser pulse have traveled enough time to a desired depth of field (herein abbreviated DOF) and have reflected back towards the sensor. At this point the shutter of the sensor is opened and photons are accumulated for a determined amount of time, substantially equivalent for all the photons of the generated laser pulse to have arrived at the desired DOF and to have reflected back to the sensor. After the predetermined amount of time has passed, the shutter on the sensor is closed and subsequent laser pulse may be emitted to the desired DOF. It is noted the terms ‘opened’ and ‘closed’ can include substantially opened and substantially closed, such that an open shutter may only accumulate a significant percentage (i.e., not necessarily 100%) of the reflected photons and a closed shutter may nonetheless accumulate a small percentage (i.e., not necessarily 0%) of the reflected photons. An open sensor may mean a relatively open sensor and a closed sensor may mean a relatively closed sensor. Thus the sensor remains closed to reflections of the laser pulse for the duration of time required for the laser pulse to travel toward a desired DOF and be reflected back to the sensor. The sensor is thus only activated for the duration of time required to receive the reflected photons of the laser pulsed from the desired DOF. In this manner, the sensor receives only reflections from the desired DOF, or at least a significant portion of the reflections from the desired DOF and avoids accumulating photons, or at least most photons from reflections of the laser pulse from objects either closer than the DOF, such as particles in the atmosphere, or farther than the DOF. The accumulated photons can then be used to generate an image of the desired DOF and any objects located therein and in this respect such systems are referred to as “gated imaging” systems, since an image is generated by only using reflections of photons from a certain DOF. Avoiding the accumulation of photons from objects in a range closer than the desired DOF is important in such systems since such objects may cause backscattering in the generated image and which can substantially reduce the contrast of objects which may be located in the desired DOF.
Gated imaging systems are usually designed with specific purposes in mind. As such, the laser used, the method of how laser pulses are generated, the characteristics of the laser pulses generated as well as the type, quality and responsiveness of the sensor used for receiving reflected laser pulses are all specifically selected. Any given real-life situation includes constraints and limitations which much be taken into consideration when designing a gated imaging system. However in many specialized fields of technology where gated imaging systems are used, these constraints and limitations may be quite loose as compared to gated imaging systems for the transportation industry. For example, in specialized medical imaging equipment or gated image laser rangefinders used in the army, the cost and weight of such systems may be substantially unrestrained. High quality expensive parts may be used to build a suitable laser for producing laser pulses, sufficient support structures may be built to house, carry and manipulate these gated imaging systems and substantially any desired laser pulse with specific characteristics for a given purpose may be designed. In addition, ultra-fast sensors may be used for imaging very specific desired DOFs. For the transportation industry however, cost, size, reliability, manufacturing simplicity and weight may become significant constraints. For example, it may not be possible to reliably achieve certain characteristics for a desired laser pulse when transportation industry quality inexpensive parts are to be used in the construction of a laser or a laser driver. And even if such characteristics can be achieved using inexpensive parts, the laser constructed might need a supporting structure, such as a specialized cooling unit, which could increase its weight, size and maneuverability significantly.
One use of a gated imaging system in the transportation industry might include an all-weather low visibility vision system installed in a consumer's vehicle. Such a gated imaging system has a number of constraints, such as a specific input voltage (from the vehicle's battery) for operating the system and a relatively low cost, for example under $1000.00. For example, a regular consumer car battery might operate in the range of 12-14 volts whereas a truck or bus might operate in the range of 24-36 volts. In addition, such a system might have requirements such as the ability to produce an optical peak power of at least a few hundred watts, for example 700 watts, in order to image sufficiently far enough ahead of the vehicle. Such systems might also need to produce laser pulses with a pulse width on the order of microseconds with a rise time and fall time on the order of hundreds of nanoseconds, or possibly on the order of nanoseconds, in order to receive sufficiently clean reflections of the generated laser pulses from objects in a scene of observation.
Reference is now made to FIG. 2A, which is a schematic illustration of a pulsed laser for use in a gated imaging system, generally referenced 50, as is known in the prior art. Pulsed laser 50 includes an electronic driver 54, a laser module 56 and an optical arrangement (not shown). Electronic driver 54 is coupled with laser module 56 via a connection 60, providing laser module 56 with a drive current suitable to produce a desired optical power (including peak power and average power) in the laser pulses produced by laser module 56, as shown by an arrow 58 which represents the produced laser pulses. The optical arrangement may be placed in front of laser module 56 for producing a focused laser pulse. The optical arrangement may also be a part of laser module 56. Both electronic driver 54 and laser module 56 may be housed on a printed circuit board (herein abbreviated PCB) 52. The coupling of electronic driver 54 and laser module 56 to PCB 52 as well as connection 60 may cause power dissipation in pulsed laser 50 thus resulting in a less efficient pulsed laser. In addition, connection 60 introduces a parasitic inductance (not shown) between electronic driver 54 and laser module 56. The parasitic inductance reduces the amount of actual current provided by electronic driver 54 to laser module 56, thus also degrading the rise time and fall time of the generated laser pulse. The power dissipation mentioned above as well as the parasitic inductance thereby reduces the efficiency of pulsed laser 50. In this respect, additional power may need to be provided by electronic driver 54 to laser module 56 to achieve a desired rise time, pulse width and fall time of the generated laser pulse.
Reference is now made to FIG. 2B, which is a schematic illustration of the pulsed laser of FIG. 2A in greater detail, generally referenced 70, as is known in the prior art. Pulsed laser 70 is to be used in a gated imaging system given the constraints and requirements listed above for a transportation consumer. Pulsed laser 70 includes a laser module 81 and an electronic driver 84. Laser module 81 is substantially similar to laser module 56 (FIG. 2A) and electronic driver 84 is substantially similar to electronic driver 54 (FIG. 2A). Laser module 81 includes a laser 76 and optics 86. Electronic driver 84 includes a power supply 72, a blocking diode 74, an active switch 78 and a timing controller 80. Laser module 81 is coupled with electronic driver 84, as shown and as was shown in FIG. 2A. In electronic driver 84, power supply 72 is coupled with blocking diode 74 and timing controller 80 is coupled with active switch 78. In laser module 81, laser 76 is coupled with optics 86. Both active switch 78 and blocking diode 74 are coupled with laser 76, thus coupling electronic driver 84 with laser module 81. Active switch 78 is coupled with a ground terminal 82.
Power supply 72 may be coupled with the power supply of a vehicle (not shown) and provides current to laser 76 via blocking diode 74. Power supply 72 may be a DC-to-DC converter. Blocking diode 74 prevents any backflow current from laser 76 from reaching power supply 72. Laser 76 receives the current from power supply 72 however laser 76 only produces laser pulses when active switch 78 is activated and laser 76 is coupled with ground terminal 82. Laser 76 thus produces laser pulses via the switching action of active switch 78. Electronic driver 84 as a whole provides a drive current to laser 76 for creating the optical power of the laser pulses generated by laser 76, as shown by an arrow 88. A focused laser pulse is produced from laser 76 via optics 86. Timing controller 80 controls the timing of active switch 78, thus controlling when laser 76 generates laser pulses. Timing controller 80 may be controlled by a sensor (not shown) of the gated imaging system (not shown) pulsed laser 70 is used with. The sensor may be a camera. In addition, timing controller 80 may be controlled by an external electronic circuit (not shown) or processor (not shown). The rate at which power supply 72 can provide current to laser 76 and the speed at which active switch 78 can be turned on and off determine certain characteristics of the generated laser pulses by laser 76, including the rise time, pulse width, fall time, number of laser pulses and laser pulse frequency of the generated laser pulses.
Reference is now made to FIG. 3, which is a graph illustrating the change in current of a laser pulse over time as generated by the pulsed laser of FIG. 2B, generally referenced 90, as is known in the prior art. Graph 90 includes an X-axis 91 and a Y-axis 92. X-axis 91 represents time in microseconds and Y-axis 92 represents current in amperes. A curve 93 shows the change in current of a laser pulse produced by pulsed laser 70 (FIG. 2B), which is also indicative of the change in optical power of the laser pulse emitted by pulsed laser 70. As shown in FIG. 3, curve 93 substantially exhibits a rise time 94 and a fall time 95 without any substantial pulse width (not labeled). The start of rise time 94 and the end of fall time 95 shows that the emitted pulse has a duration of about 1.3 microseconds, having a rise time close to 1 microsecond. As described above, such a pulse does not fit the suitable characteristics for a gated imaging system for the transportation industry where a pulse width on the order of microseconds is desired along with a rise time and fall time on the order of hundreds or even tens of nanoseconds. A sufficiently fast rise time and fall time is desired in order to avoid blurring in images generated from reflections of the generated laser pulses from objects in a scene of observation. In addition, a sufficiently fast rise time and fall time will enable better three-dimensional accuracy of gated imaging systems such as laser range finders and time-of-flight sensors, better visibility performance in inclement weather (e.g., smog, rain, snow, fog and the like) and a shorter and sharper DOF.
What is thus desired is a pulsed light source for use in a gated imaging system which suits the constraints and limitations of the transportation industry. As mentioned above these constraints and limitations include sufficient optical power to image at least a few hundred meters, the ability to use transportation industry quality parts in constructing the pulsed light source, a light pulse having a sufficiently fast rise time and fall time on the order of hundreds or even tens of nanoseconds, with a pulse width on the order of microseconds, as well as being cost effective.